Observations of powerful radio galaxies, when combined with synchrotron spectral ageing models, show good agreement with the scenarios described above (e.g. [1]}, [2]}, [3]}). However, in a small number of high power objects (\(L_{\rm 178~MHz} > 10^{27}\)  W Hz\(^{-1}\)  sr\(^{-1}\) ), the hotspot radio spectra are seen to flatten towards low frequencies (\(<\)  327 MHz; e.g. [4]}), with the spectrum becoming flatter than the canonical \(S_\nu \propto \nu ^{-0.5}\)  spectrum predicted for diffusive shock acceleration [5]}. This flattening has been generally interpreted as a low-energy cut-off (LEC) in the energy distribution of the relativistic particles within the hotspots, where a spectral turnover is expected to be seen at low frequencies. However, this has never been observationally verified due to the limited spectral coverage and angular resolution of low frequency radio telescopes.
